A microwave plasma processing apparatus known in the related art, which processes a processing target substrate using high-density plasma excited by microwaves. The microwave plasma processing apparatus radiates microwaves into the processing apparatus using, for example, a flat plate type antenna having a plurality of slots arranged therein. Thus, more uniform microwaves may be generated in the processing apparatus. Accordingly, the microwave plasma processing apparatus may generate more uniform high-density plasma by ionizing the gas supplied into the processing apparatus.
Further, a dielectric window made of a dielectric is provided on the bottom surface of the antenna. A rib is formed on the dielectric window to secure a mechanical strength. See, for example, Japanese Patent Laid-Open Publication No. 2015-018686.